Method of bonding a bi-metallic casting



Dec. 18, 1962 A. F. BAUER METHOD OF BONDING A BI-METALLIC CASTING Filed July 16, 1958 2 Sheets-Sheet l 24 INVENTOR Alfred F. Bauer TTORNEY Fig. l.

Dec. 18, 1962 A. F. BAUER 3,

METHOD OF BONDING A BI-METALLIC CASTING Filed July 16, 1958 2 Sheets-Sheet 2 Fig. 3.

INVENTOR Alfred F. Bauer i ga ATTORNEY United States Patent Oflfice 3,069,209 Patented Dec. 18, 1962 3,069,209 METHOD OF BONDING A BI-METALLIC CASTING Alfred F. Bauer, 2108 Parkside Blvd., Toledo, Ohio Filed July 16, 1958, Ser. No. 749,017 11 Claims. (Cl. 309-3) This invention relates to the production of a bi-metal lio bond. More specifically, it is concerned with the production of an article comprised of two dissimilar metals in intimate contact. This invention also relates to a method of fabricating articles of manufacture of bi-metallic structure wherein excellent heat transfer characteristics are developed between the dissimilar metals and through the metallic interface. The invention further contemplates bi-metallic articles of manufacture having unusually high strength characteristics.

The automotive industry is illustrative of the changing trend to lightweight metals. This change has been due, in part, to the attempt to reduce the ever-increasing Weight of automotive vehicles. Another reason for the change is the desire for better heat flow in certain parts, such as, for example, the brake drum and engine.

Previously, brake drum assemblies were constructed of iron or steel, which materials not only possessed the required strength characteristics but also were eminently suitable as materials of construction where friction problems were encountered. It was recognized that cast iron and/or steel did not possess the most satisfactory heat transfer capabilities; however, until recently, dissipation of heat during braking did not present too great a problem. With engines of higher horsepower resulting in greater speeds and with autos now weighing more than ever, the inertia which must be overcome in stopping a car is so great that a tremendous amount of heat is generated. The heat fiow characteristics of ferrous metals are not adequate to dissipate this amount of heat quickly with the result that brake fade or failure occurs. There are many light-weight metals such as, for example, aluminum, magnesium and alloys of these metals which have much better heat transfer characteristics than do iron and steel. Such metals, however, generally do not possess the toughness or strength necessary for frictional operations as encountered in a brake drum. As a compromise, it was, therefore, proposed that such an assembly be fabricated as a bi-metallic unit; it being desirable, as a typical example, that one employ a cast iron or steel insert for the braking surface thereby utilizing the desirable characteristics of iron or steel in the frictional operation. A light-weight metal such as aluminum would then be utilized to form the main body of the assembly thus utilizing the desirable characteristic of improved heat transfer through the lighter metal.

Another use for such a bi-metallic assembly is for cylinders of an internal combustion engine. The aluminum, because of its greater thermal conductivity, conducts the heat away from the combustion chamber more efficiently than the gray iron which was formerly used. Aluminum has been found to be unsuitable for the interior surface of the cylinder walls since it will not withstand the rubbing action of the piston rings and a gray iron liner has been incorporated to provide the desirable wear-resistant properties.

The production of these castings and others which, for proper operation, require uniform and efficient heat flow has plagued manufacturers for many years. Efforts have been made to cast-metals such as aluminum around iron or steel inserts by conventional casting techniques such as, for example, sand casting and permanent mold. While assemblies produced by conventional techniques have 'found limited acceptance, there have been certain ob- 'vious drawbacks which have prevented exploitations for 0 v 5 .sary that the outer surface of the insert, which may be such assemblies to the fullest possible capacity. The production of bi-metallic assemblies according toprior art techniques has often resulted in the formation of a mechanically weak bond between the metals.

The flow of heat through bi-metallic assemblies produced by conventional casting techniques is usually sufficient at normal temperatures but its efficiency is quite low at the higher temperatures encountered in operation. The thermal expansion coefficients of the two metals comprising the assembly are unequal, that of the aluminum being approximately twice that of the gray iron, and at the relatively high operating temperatures during the braking operation or during the running of an engine, the aluminum tends to expand to a much greater extent thanvdoes the gray iron. As the contact between the two metals becomes less intimate, the flow of heat diminishes until, in those cases where the aluminum expands enough to form voids at the interface, thermal barriers are created which may act as insulators against the flow of heat.

Accordingly, an object of this invention is the production of a bi-metallic bond. Another object is the production of a fabricated assembly comprising two dissimilar metals. Another object is to provide a method of effecting a strong mechanical bond between two dissimilar metals. Still another object is to produce an article of bi-metallic composition having excellent heat transfer properties. These and other objects of the instant invention will become apparent from the following more complete description and from the examples. Broadly, the instant invention covers a method for fabricating a bi-metallic article which comprises steps of pressure die casting a mass of light-weight metal around a dissimilar metal insert.

As hereinafter employed, the phrase die casting is intended to mean pressure die casting. Pressure die casting may broadly be defined as the technique of introducing molten metal under high pressure into a closed die of desired configuration. Typical pressures thus employed may range from about 4,000 to about 10,000 p.s.i.

This is to be distinguished fromconventional casting operwherein the only pressure encountered is due to the hydrostatic pressure of the metal in risers provided in the molds. Pressure casting operations are widely employed to produce articles from many light-weight metals such as, for example, aluminum, magnesium, zinc and alloys thereof,

.In order to produce a satisfactory bi-metallic bond according to the instant invention, it has been found necesenter. It is estimated that the irregular protrusions and interstices forming the undercuts will give an increase in surface area some four to five times that of a machined surface of a corresponding insert. It has not been determined that the depthof the interstices or the height of the protrusions is.critical; however, excellent results have been obtained when such depth or height is in the .magnitudeof .040.060 inch. It is possible to produce a metallic insert having the desired surface characteristics by various methods known to the art. For example, one method which has been found particularly acceptable is that which employs centrifugal casting techniques and is described in the patent to Myers 2,623,809 which issued December 30, 1952. In most molding operations it is 'customary to coat the mold with a refractory composition so as to protect the mold and also to enable easy removal of the casting from the mold. These refractory compositions, which are usually applied to the mold surface in the form of fluids, are extremely porous when dry and, as a result, the metal which forms the insert casting, while in the molten state, flows into the pores of the mold coating. The result is that a roughened surface, such as that required in order to practice the instant invention, is produced as shown and described in the Myers patent above mentioned.

According to the process of the instant invention, an insert assembly of the type just described is inserted into the die of a pressure die casting machine, the die cavity of the die having the desired configuration of the article to be cast. Typically, the insert is heated to the operating temperature of the die prior to introduction of the molten metal component. The molten metal, for example aluminum, is then forced into the die cavity under conditions of extremely high pressure between 4,000 and 10,000 p.s.i. The molten metal is caused to flow with ease into the interstices on the surface of the dissimilar metal insert. As previously stated, these interstices are typically .040 to .060 inch deep. It is only by employment of the extremely high pressures which are developed in pressure die casting technique that the molten metal will penetrate and completely fill interstices of this small size. The molten metal is then permitted to cool and solidify around the dissimilar metal insert. The result of this operation is the formation of an interlocking bond between the two metal constitutents. The presence of literally thousands of protrusions and interstices results in the formation of much more contact area than is possible with conventional casting techniques. When one employs conventional casting techniques, such as permanent mold or sand casting, the hydrostatic pressure is not great enough to force the molten metal completely into the interstices and, therefore, a bond will result which is not only inferior in mechanical properties but also in heat transfer characteristics. It is postulated that the high pressures employed in pressure casting operations overcomes the surface tension of the molten aluminum and forces same into even the most minute openings in the insert surface.

In the drawings:

FIG. 1 is a sectional view of a representative casting which may be produced by the method of this invention;

FIG. 2 is a photographic view of an enlarged portion of the surface of the insert or liner;

FIG. 3 is a photographic view of an enlarged sectional portion of the bi-metallic casting;

FIG. 4 is a perspective view, partially sectioned, of a cylinder sleeve for an internal combustion engine which may be produced by the instant process; and;

FIG. 5 is a sectional view of an engine block for a small internal combustion engine.

A bi-rnetallic brake drum as shown in the drawings has been chosen as being representative of the type of casting which is especially adapted to production by the method of this invention. While the following description will be concerned with the casting of a specific article, such limitation is for the purpose of brevity and clarity and is not intended to limit the scope of the invention.

The brake drum of FIG. 1 is die cast of light-weight metal such as aluminum or magnesium and comprises annular flange 12, web 14 and hub 16. Liner is in the form of a hollow cylinder which is placed in the die cavity as an insert around which the aluminum is pressure die cast. This insert is preferably made of gray iron but any other material having the desired frictional properties may be used.

The gray iron liner 20 as employed in this example is made by centrifugal casting in a cylindrical mold. The mold is first coatedwith a mold coating, the composition and application of which may be according to any of several known methods. This coating is of a nature such that the exposed surface of the coating is roughened either by pores or cavities in the coating or by protuberances projecting toward the center of the mold. The coating is also of a nature such that it will not wash away when the molten metal is introduced into the mold but rather will maintain its roughened surface and will impart a complementary surface to the exterior of the casting.

Any of the mold coating which adheres to the casting when it is taken from the mold is removed in such a way that the casting retains its rough spiny exterior. Removal of the coating may be accomplished by an electrolytic salt bath or by a light sand blasting or any of numerous other ways known to those skilled in the art. FIG. 2 shows a portion of the outside surface of a typical liner enlarged approximately ten times. The protrusions and interstices are a result of the rough surface of the mold coating. The metal as it is being cast is forced into all the voids and cavities of the coating. Upon solidification the outer surface of the casting has a surface which is complementary to that of the coating. It is preferred that the protrusions be somewhat mushroomed in shape and the interstices form undercuts, i.e., that the protrusions be smaller at the surface of the liner and get larger as they extend therefrom'and the interstices, between the protrusions, be conversely shaped. The casting is then cut to the desired length to be used as a liner.

The liner is preferably heated before being inserted into the die casting die. It is preferable to have the temperature of the insert at least as high as the operating temperature of the die so that the molten metal will, upon being injected into the die, be forced into all the interstices of the liner surface before it starts to solidify.

The liner is placed onto the core plug in the die and the die is closed. The molten metal is then injected into the die cavity to form the brake drum. Since the metal is injected into the die under a pressure of from 4,000 to 10,000 pounds per square inch, it enters the smallest crevice in the rough surface of the liner. As the cast metal solidifies it becomes interlocked with the insert in the multitude of irregularly-shaped protrusions and interstices. These interlocking surfaces cover substantially the entire interface of the two metals. FIG. 3 is a photograph of the cross-section of a typical casting, enlarged ten times, in which the darker portion is the gray iron insert and the lighter portion is the aluminum. The interlocking action between the die cast aluminum, which has been forced into the interstices, and the protrusions projecting from the surface of the insert is readily seen in the photograph.

This mechanical bi-metallic interlocking bond between the ferrous liner and the die cast aluminium is sufficient to maintain the two metals in intimate contact at all points without the need for providing a chemical or metallurgical bond of any kind. The present method represents a great saving in time, material and labor over previous methods of producing similar articles.

The rough surface of the liner provides much more surface which is in contact with the aluminum than is possible with an insert having a relatively smooth surface. The increased surface area of the liner provides much more effective heat transfer from the braking surface to the aluminum and no thermal barriers can be formed since the two metals are so interlocked that they will not be separated by any difference in expansion of the two metals as they become heated.

The bi-metallic interlocking bond which is produced with the above described method has been found to exist only when the molten metal is introduced into the die under relatively high pressures such as employed in die casting techniques. This type of bond is not present when the casting is made by sand casting or permanent mold processes since the molten metal does not enter the interstices between the protrusions on the liner but rather 30, as in FIG. 5.

bridges over them leavingthe' interstices filled with air to act as an insulating medium.

The instant process may be employed to produce any other articles of manufacture wherein the body of the article is pressure die cast around a liner or insert of dissimilar metal. The primary stipulation is that the melting point of the metallic liner be higher than that of the die cast metal so as to prevent a softening of the liner surface while it is in contact with the molten metal as it is injected into the die. Such an article is exemplified by a cylinder member for an internal combustion engine which may be in the form of a separate sleeve 20a, as shown in FIG. 4, or an integral part of the engine block It is customary to form the body of such cylinders of a metal such as aluminum or magnesium having a high thermal conductivity and provide a ferrous metal liner to furnish the necessary wear-resistant properties for the cylinder bore.

The separate sleeve member 20a comprises a body portion 22 of lightweight metal which has been pressure die cast around liner 24. The engine block 30 of FIG. includes cylinder 32 cast as an integral part thereof. A ferrous metal liner 24 forms the bore of the cylinder which is subjected to the abrasive action resulting from the reciprocating motion of the piston and piston rings (not shown). The exterior surface of liner 24 has a vast multitude of irregular protrusions and interstices which provide a means for the intimate interlocking of the ferrous liner and the die cast metal throughout the entire interfacial surface.

While a liner for a cylinder of an internal combustion engine would not be subjected to the torque and rotational stresses encountered in a brake drum, the heat transfer requirements for a cylinder equal or surpass those for a brake drum. Uniform heat transfer, such as results fom the bi-metallic interlocking bond produced by the instant process, is necessary to prevent the occurrence of hot spots in-the cylinder where the lubricant could burn away and allow the piston to freeze.

The specific examples given employ an insert or liner which is cylindrical in shape; however, the instant process is by no means limited to such applications. Pressure die castings having dissimilar metal inserts or liners of widely varied configuration bonded to the die cast metal may be produced by this process.

The instant process may be carried out either on conventional pressure die casting machines or on pressure die casting machines whichemploy vacuum techniques for evacuating air from the die cavity.

Various tests have been conducted to compare the quality of the bond obtained by the instant process with that of numerous known methods. For one of the tests, bimetallic brake drums were obtained which incorporated three different bonding methods, namely: (1) -a well known chemical or metallurgical bonding method whereby an alloy of the two metals is formed at the inter-face; (2) a mechanical bonding method wherein a portion of the metal is cast into dovetail-shaped grooves which have been machined into the exterior of the liner; and (3) the bi-metallic interlocking bond of the instant method. It should be noted that all of the brake drums tested comprised a gray iron liner and an aluminum alloy body. Similar test pieces were cut from the annular portion of each brake drum.

For the tension tests, the test piece was clamped in a fixture so that the load wasapplied in a direction substantially perpendicular to the general plane of the interfacial bond. The test pieces were mounted in the fixture for the shear test in such a way that the load was applied in a direction substantially parallel to the interfacial plane. Both of these tests were conducted on a standard Tinius- Olsen testing machine. Several pieces from each drum were tested and the averaged results are as follows, giving the load required to break the bond.

6 Shear Test P.s.i. Method 1 wherein a chemical bond is formed 5,400 Method 2 which employs dovetail-shaped grooves 6,850 Method 3 which is the bi-metallic interlocking bond 9,530

Tension Test P.s.i. Method 2 3,420 Method 3 6,480

Because of the lack of gripping surface, tension tests could not be conducted on pieces cut from the brake drum bonded by Method 1. From a comparison of these test results it is quite apparent that the bi-metallic bond obtained by the method of this invention is far superior to that obtained by presently known methods.

While specific examples have been given to illustrate the instant invention, these examples should not be considered as limiting the scope of the following claims.

I claim:

1. The method of producing a bi-metallic casting which comprises the steps of providing a metal centrifugally cast ferrous liner having one rough surface containing a multitude of irregular protrusions and interstices, inserting said liner in the die cavity of a pressure die casting machine such that the rough surface is exposed, injecting into the die cavity under high pressure a molten lightweight metal, exerting sufiicient pressure upon said molten metal to force it into the interstices of the liner surface thereby filling said interstices, cooling the molten metal and solidifying it whereby the solidified metal in the interstices of the liner becomes intimately interlocked with the irregular protrusions on the liner.

2. The method of producing a bi-metallic brake drum which comprises the steps of providing a generally cylindrical metal liner having an outside surface covered with a multitude of irregularly shaped protrusions and interstices, placing said liner on a core member located within the die cavity of a pressure die casting machine, injecting into the die cavity under pressure a molten metal having a melting point lower than that of said liner, continuing to exert sufiicient pressure on said molten metal to force it into the interstices of the liner thereby filling said interstices, cooling and solidifying the molten metal whereby the die cast metal in the interstices becomes intimately interlocked with the protrusions on the liner.

3. The method of producing a cylinder sleeve for an internal combustion engine which comprises the steps of providing a generally tubular ferrous metal liner having an outer surface covered with a multitude of irregular protrusions and interstices, placing said liner on a core member positioned within the die cavity of a pressure die casting machine, injecting into the die cavity under high pressure a molten metal having a melting point lower than that of said liner, continuing to exert sulficient pressure on the molten metal to fill said interstices of the liner, cooling and solidifying said molten metal whereby the metal in the interstices becomes intimately interlocked with the protrusions on the liner.

4. A bi-metallic brake drum comprising a generally cylindrical ferrous metal liner having a bore which presents a frictional braking surface and an outer peripheral surface covered with a multiplicity of irregular protrusions and interstices, and a pressure die cast body of dissimilar metal including (1) an annular flange surrounding said liner and in intimate contact therewith throughout the interfacial area between the two dissimilar metals, said die cast metal filling the interstices and surrounding the protrusions of said liner, (2) a hub member and (3) a web portion interconnecting said flange and hub, the two dissimilar metals being mechanically bonded by the interlocking of the die cast metal in said interstices with the protrusions in the liner.

5. A bi-metallic brake drum as defined in claim 4 wherein the die cast metal is an aluminum alloy.

6. A pressure die cast engine block for an internal combustion engine including integral cylinder members comprising cylindrical ferrous metal liners having a smooth bore which provides a wear-resistant surface and an outer surface covered with a multiplicity of irregular protrusions and interstices of the type produced by centrifugal casting, and pressure die cast light-eight metal cylinder walls surrounding said liners and in intimate contact therewith throughout the interfacial area between the two dissimilar metals, said die cast metal filling the interstices of the liner, the two dissimilar metals being mechanically bonded by the interlocking of the die cast metal with the protrusions on the liner.

7. A cylinder sleeve for an internal combustion engine comprising an annular pressure die cast light-weight metal wall portion and a tubular liner of dissimilar metal whose melting point is higher than that of the die cast metal having a bore presenting a smooth, wear-resistant surface and an outer surface which is covered with a multiplicity of irregularly shaped protrusions and interstices, said die cast wall portion surrounding said liner and in intimate contact therewith throughout the interface between the dissimilar metals, the die cast metal filling the interstices of the liner and the two dissimilar metals being mechanically bonded by the interlocking of the die cast metal in said interstices with the protrusions on the liner.

8. The method of producing an engine block for an internal combustion engine which comprises the steps of providing a centrifugally cast generally tubular metal liner having an outer surface covered with a multitude of irregularly shaped protrusions and interstices, placing said liner on a core member positioned within a die cavity of a pressure die casting machine, injecting a molten metal having a melting point lower than that of said liner into the die cavity over a pressure sufiicient to force the molten metal into said interstices of the liner, cooling and solidifying said molten metal to form a casting whereby the metal in the interstices becomes intimately interlocked with the protrusions on the liner.

9. The method of producing a bimetallic casting comprising the steps of positioning in an appropriate die a previously centrifugally cast metal body having a pitted and undercut surface containing a multitude of irregular protrusions and interstices, die-casting under high pressure a dissimilar molten metal into the die and into intimate contact with substantially all parts of the pitted and undercut surface, and solidifying the die-cast metal, whereby in the bimetallic casting produced, the metal body is mechanically locked to the die-cast metal, and the mechanical locking is effective to prevent separation of the dissimilar metals by a relative force acting normal to the pitted and undercut surface.

10. A circular bimetallic article consisting of an outer die cast light metal mufi firmly attached to an inner circular, centrifugally cast body composed of a metal selected from the group consisting of ferrous and cuprous metals, said centrifugal casting having an as-cast barnaclelike surface on at least that portion of the casting in contact with the light metal die casting, said castings being mechanically interlocked at their interface by the light metal filling the recesses in the said barnacle-like surface of the centrifugal casting.

11. A pressure cast bimetallic article composed of a cast wear resistant member of a metal selected from the group consisting of ferrous and cuprous metals and a cast light metal heat dissipating member having a substantially uniform structure, said wear resisting member having barnacle-like projections on at least the surface in contact with the light metal resulting from casting said member in contact with a surface having barnacle-like recesses, said light metal member being interlocked with said barnacle-like surface, the light metal filling the recesses in said surface, said bimetallic article being characterized by a firm attachment between the two metal members resistant to loosening under repeated heating and cooling cycles.

References Cited in the file of this patent UNITED STATES PATENTS 2,046,369 Dake July 7, 1936 2,085,727 Campbell July 6, 1937 2,331,554 Irgens Oct. 12, 1943 OTHER REFERENCES Die Casting for Engineers, by the New Jersey Line Company, pub. 1942. (Copy in Div. 3 and Scientific Library, pages 1 and 129 relied on.) 

7. A CYLINDER SLEEVE FOR AN INTERNAL COMBUSTION ENGINE COMPRISING AN ANNULAR PRESSURE DIE CAST LIGHT-WEIGHT METAL 